The extruder is generally equipped with a hopper to which a resin material is fed, a screw by which a material is transported, kneaded and pressurized, a cylinder (a barrel) having a function of externally heating and cooling the resin material and a vent by which gaseous volatile components are discharged and the like, and a die is installed at the tip of the extruder. In order to produce a molded article by using such extruder, the resin material is first fed into the hopper and the fed resin material is transported to the front by the rolling screw and is heated from the cylinder side to melt gradually and gaseous volatile components contained in the melted resin material are discharged from the vent, followed by pressurizing and passing the melted resin material through the die to mold.
In producing a molded article by an extruder, an additive such as a stabilizer is blended into a resin and the additive is usually mixed with a resin material and the mixture is fed from a hopper. However, part of the additive kneaded with the resin material in the extruder was sometimes discharged from a vent hole, which was uneconomical. Further, kneading of a resin material and an additive was sometimes insufficient depending on the kneading conditions.
As a technique to solve such problems, in Japanese Patent Laid-Open Publication No. 2000-202887, there is disclosed a process for producing a resin composition in which a liquid additive is added to a thermoplastic resin. In said production process, there is disclosed a process in which the liquid additive is added from a feed opening provided between a final vacuum vent and a die and further there are provided functions of counterflow of the additive to the vent and distribution mixing. In said Japanese Patent Laid-Open Publication, a preferable range of L/D between the additive introduction opening and the die is set in the range of from 5 to 10.
While, since a cycloolefin resin is frequently applied to optical members utilizing the characteristics of transparency, favorable optical characteristics are demanded. However, when the similar addition method as described in Japanese Patent Laid-Open Publication No. 2000-202887 was performed for the purpose of blending various additives with a cycloolefin resin, there was a problem that favorable optical characteristics were not maintained if the L/D between the additive introduction opening and the die is in the range of from 5 to 10.
Further, the water absorption coefficient of a cycloolefin resin is extremely low, and the water which has been contained in a resin during high temperature and high humidity testing coagulates, sometimes leading to the occurrence of fine cracking. As a means to solve this problem, a hydrophilic additive is required to be added, however, as mentioned above, there was a problem that if the hydrophilic additive was added by a known method, favorable optical characteristics were not maintained.